Ink jet printing is a non-impact method that in response to a digital signal produces droplets of ink that are deposited on a substrate such as paper or transparent film. Ink jet printers, especially thermal or bubble jet drop-on-demand printers have found broad application as output for personal computers in the office and the home.
Thermal ink jet printers use a battery of nozzles each containing a resistor element to fire ink droplets toward the print media. With aqueous inks, generated water vapor under pressure expels the droplets of ink. In many versions of thermal printers, a battery of nozzles is contained in a disposable ink cartridge which carries a supply of ink sufficient to allow each nozzle to print many millions of drops. Therefore it is critical that the thermal resistor function uniformly over the long life of the cartridge, i.e., fire millions of drops without a change in heat flux to the ink. Print quality is greatly affected by the degree of heat flux.
Early ink jet devices used organic solvents based inks. In office and home applications these have been largely replaced by primarily water based inks which are safer and environmentally more compatible. These inks are colored most readily with water soluble dyes which have been perfected to resist plugging nozzles.
These dyes have some serious deficiencies. They remain soluble in water on the printed page are therefore subject to leaching by water, for example rain water and coffee. This deters use of ink jet printers in applications such as envelop addressing. Moreover, many of the dyes in use have poor lightfastness and fade even on exposure to fluorescent lighting used in offices.
Pigmented ink jet inks have lightfastness and water fastness advantages over dye-based counterparts, provided the pigment dispersions can be stabilized to flocculation, aggregation or settling.
One major drawback of pigmented inks in thermal drop-on-demand ink jet printers is the formation of a pigmented film on the resistor surface during prolonged printing. The result of this film is the steady and continuous loss of heat transmission to the ink resulting in a steady reduction in ink drop velocity and volume. The phenomena is termed "deceleration". As film builds on the resistor it insulates the surface. This causes a degradation in optical density at normal printing frequencies with the resultant degradation in print quality. When printing a large solid pattern this will result in a loss of optical density as the printer proceeds from the top to the bottom of the page. This phenomena allows for the measurement of deceleration as a degradation in density with prolonged pen firing. (See Section II of Examples). In cases where the situation is severe one observes deceleration as a gradual loss of optical density or fading across a printed line of text. In extreme cases the pen will totally fail to print. In less severe cases where printing can be carried out to millions of drops, the formed film may undergo thermal decomposition which eventually results in resistor malfunction.
A second important requirement for a pigmented ink is freedom from nozzle pluggage. In the case of pigmented inks this can arise from flocculation of the pigment dispersion.
Cosolvents or additives that prevent or retard deceleration must also be compatible with the pigment dispersion and allow for freedom of operation without pluggage. They must also not promote pigment settling as this will produce variation in print quality with the age of the ink cartridge.
Accordingly, a need exists for cosolvents or additives which eliminate film formation on thermal resistor surfaces without destabilizing pigment-based inks.